Correlation between CAM-Cycling and Photosynthetic Gas Exchange in Five Species of Talinum (Portulacaceae)

Abstract

Photosynthetic gas exchange and malic acid fluctuations were monitored in 69 well-watered plants from five morphologically similar species of Talinum in an investigation of the ecophysiological significance of the Crassulacean acid metabolism (CAM)-cycling mode of photosynthesis. Unlike CAM, atmospheric CO2 uptake in CAM-cycling occurs exclusively during the day; at night, the stomata are closed and respiratory CO2 is recaptured to form malic acid. All species showed similar patterns of day-night gas exchange and overnight malic acid accumulation, confirming the presence of CAM-cycling. Species averages for gas exchange parameters and malic acid fluctuation were significantly different such that the species with the highest daytime gas exchange had the lowest malic acid accumulation and vice versa. Also, daytime CO2 exchange and transpiration were negatively correlated with overnight malic acid fluctuation for all individuals examined together, as well as within one species. This suggests that malic acid may effect reductions in both atmospheric CO2 uptake and transpiration during the day. No significant correlation between malic acid fluctuation and water-use efficiency was found, although a nonsignificant trend of increasing water-use efficiency with increasing malic acid fluctuation was observed among species averages. This study provides evidence that CO2 recycling via malic acid is negatively correlated with daytime transpirational water losses in well-watered plants. Thus, CAM-cycling could be important for survival in the thin, frequently desiccated soils of rock outcrops on which these plants occur.

Canopy CO₂ concentrations and Crassulacean acid metabolism in Hoya carnosa in a subtropical rain forest in Taiwan: consideration of CO₂ availability and the evolution of CAM in epiphytes

“…The open gas exchange system comprised: a LI-COR LI-6262 differential infrared gas analyzer; polycarbonate, waterjacketed gas exchange cuvettes with small fans for air mixing; thermocouple and thermistor temperature sensors and meters for air and leaf temperature measurements; a temperature-controlled humidifier; and a computer for data collection. Further details of this system, as well as methods of data analysis, have been described in Harris and Martin (1991) and Gravatt and Martin (1992).…”

C 3 Photosynthesis in the Gametophyte of the Epiphytic CAM Fern Pyrrosia longifolia (Polypodiaceae)

“…Gas exchange-Gas exchange was measured with an open-flow, differential infrared gas analyzer system as described in Harris and Martin (1991) and Gravatt and Martin (1992). Environmental conditions within the cuvettes were: 250-300 ^mol m~2 sec" 1 PPFD for 12 hours, 20 "%8.8…”

Crassulacean Acid Metabolism in Three Species of the C4Genus Portulaca

“…Whether CAM-cycling directly benefits a plant is unclear. Several hypotheses have been posed, including a possible increase in water conservation (Martin et al 1988;Harris and Martin 1991) and a potential reduction of photo-inhibition (Osmond et al 1980;Osmond 1982). Accumulating evidence, however, suggests that CAM-cycling may simply be a precursor to "CAM-idling" and may not directly benefit the plant (Rayder and Ting 1981;Sipes and Ting 1985;Martin 1995).…”

“…C0 2 and water vapor exchange were measured with an open, differential infrared gas analysis system (Harris and Martin 1991;Gravatt and Martin 1992). Intact shoots were sealed in gas exchange cuvettes for 2 d; soil and roots remained outside the cuvettes.…”